Fluid analysis is used in a wide variety of research, manufacturing, waste disposal and other applications. Typically, the fluid is transported from one location to another in a fluid stream, and it is often desirable to analyze the contents of the stream. This may be done by taking a sample from the fluid stream at selected time intervals. However, the composition of the fluid stream may vary considerably between the sampling intervals, so that the samples may not accurately reflect the composition of the stream. Collecting samples from the stream is also physically complicated. It is therefore often preferred to link an analyzer in-line with the fluid stream. Generally, analyzers only need test a small fraction of the total quantity of a stream to function properly, so that the majority of the fluid stream may bypass the analyzer. Diverting a small fraction of a fluid stream to an analyzer without using extensive and complicated plumbing can be a considerable design challenge.
Along with the fluid stream, it is necessary to supply a known fluid sample to an analyzer from time to time to insure that the analyzer is functioning properly or to calibrate the analyzer. This is done by observing whether the analyzer correctly analyzes the known sample. If the analyzer does not correctly identity the known sample, then the analyzer must be calibrated until it is operating within desired parameters. Of course, if the known sample does not in fact contain the precise composition that it is believed to contain, the analyzer will be incorrectly tested and calibrated. Therefore, it is imperative that the known sample be delivered to the user in a controlled condition. An ideal method of ensuring this is for the known sample to be prepackaged in small containers such as vials in the controlled environment of a facility specializing in such matters. This is especially an issue in the pharmaceutical industry where regulations require the measurement of total organic carbon in waters for injection and purified waters at levels below 500 ppb. Standards with such low concentrations are difficult to produce without special laboratory equipment that isolates the standard from carbon that may be in the air, and the containers for such standards must be cleaned with special cleaning agents and rinse water with ultra-low carbon concentrations.
Supplying the known sample to the analyzer may complicate the process of analyzing the fluid stream. Analyzers, in general, have only one inlet, so that the known sample must somehow be substituted into and out of the fluid stream path when the analyzer is tested. The prior art method of performing this swapping involves a multitude of discrete steps, many of which require plumbing changes and special tools. First, the fluid stream must be disconnected from the analyzer. Next, the known sample must be connected to the analyzer. After the analyzer is tested and calibrated, the known sample must be disconnected from the analyzer, and the fluid stream must be reconnected. Depending on the application, the process of connecting and disconnecting the fluid stream and the known sample may need to be repeated many times over relatively short time frames, such as days or weeks. As well as requiring much time from the operating personnel, the process causes substantial down time to the flow of the fluid stream, which may interfere with the manufacturing, disposal or other operations involving the fluid stream. The connecting and disconnecting process causes substantial wear on the involved parts, and hence reduces their reliability and operating life. A mistake in this process may allow fluid to leak from the fluid stream or the analyzer, perhaps endangering the health of persons exposed to the leaked fluid and certainly disrupting whatever downstream applications are in use. The use of valving or tubing sections that are unique to either the known sample or to the fluid stream adds complexity and moving parts, and invites contamination that can produce inaccurate or imprecise results. In the case of the fluid stream, this contamination can lead to the disruption of the downstream application of the fluid stream, potentially fouling manufacturing or treatment equipment. In the case of the known sample, contamination will result in an inaccurate sample being supplied to the analyzer, resulting in the analyzer being improperly tested and calibrated. When the fluid stream is once again connected to the analyzer, it may be erroneously analyzed by the improperly calibrated analyzer.
The above complications may prevent fluid stream operators from testing and calibrating an analyzer as often as may be optimally desired, or even to entirely forgo testing and calibrating the analyzer on any sort of a regular basis whatsoever. Simplifying the method of diverting a portion of a fluid stream supplying a known sample to an analyzer would therefore greatly improve the ease and efficacy of fluid stream testing. It is also very desirable that the apparatus be capable of operating at low flow rates. Thus, the interior dimensions of the apparatus should be small, to allow low flow rates while still ensuring rapid response times.
One approach to the problem is to maintain a store of known samples for calibration purposes inside the apparatus. The known sample would then be drawn upon each time the apparatus is calibrated by means of suitable switching and valving. A major drawback to that approach, however, is that many samples are not stable over time; therefore, the sample being used for calibration could have deteriorated at the time of the calibration in a manner that produces an erroneous calibration. Further, many samples used for calibration cannot be stored indefinitely within the apparatus due to safety concerns or, especially with respect to low level samples, contamination concerns.
Examples of compounds that are often tested for in fluids are those containing sulfur, nitrogen, and organic and inorganic carbon. For instance, a carbon detector that may benefit from the present invention is described is U.S. Pat. Nos. 5,443,991 and 5,132,094 to Godec et al. and assigned to the assignee of the present invention. Such detectors measure total organic carbon concentration (TOC) and total carbon concentration in water, a standard method for assessing the level of contamination of organic compounds in potable waters, industrial process waters, and municipal and industrial waste waters. TOC measurement is used to determine the purity of potable and process water for manned space based systems such as the space shuttle, and will in all likelihood be used in future manned explorations. A detector according to an embodiment of the above mentioned patents includes an acidification module, an inorganic carbon removal module incorporating a gas permeable membrane, and an oxidation reaction system. Coupled with an oxidation reactor to form carbon dioxide and a high sensitivity conductomeric detector, such a detector allows on-line measurements of the TOC of aqueous streams. Other carbon detection systems suitable to analyze aqueous streams use IR spectroscopy instead of conductomeric techniques.
A process for determining sulfur containing compounds (as well as other compounds) in a fluid stream is described in U.S. Pat. No. 5,310,683 to Godec et al. and assigned to the assignee of the present invention. Sulfur detection is used in diverse industries such as petrochemical refining, beer brewing and other consumer product manufacturing. In consumer products, trace levels of sulfur-containing compounds can impart an objectionable taste and odor. In petrochemical applications, trace sulfur contaminants can poison catalysts, damaging or destroying processing equipment. Constant and accurate monitoring of fluid streams in these processes is therefore an economic necessity. The method described in the above patent involves combusting a sample in a hydrogen/air flame of a flame ionization detector, and measuring the ionic species produced in the flame. Concurrently, sulfur monoxide produced in the flame is withdrawn and measured by ozone-induced chemiluminescence. Both sulfur monoxide, from which sulfur containing species are measured, and ions containing carbon, from which organic compound concentrations can be deduced, are formed in one detection operation.
The above analyzers are representative of the broad class of analyzers used to monitor fluid streams. The present invention has application with a wide variety of other analyzers designed to detect and measure many different compounds.